In the manufacture of semiconductor devices, leadframes are commonly used as supports for mounting and processing a plurality of semiconductor dice or chips concurrently during what is generally referred to as semiconductor packaging. After a chip is mounted on a leadframe, electrical connections are made between the chip and the leadframe. The chip and part of the leadframe are then encased by plastic molding compound, such as epoxy molding compound (EMC) to form an electronic package. The molded electronic package may then be mounted to electrically connect the semiconductor chip to an external circuitry.
FIG. 1 is an elevation view of a leadframe device 100 that has been partially molded with an epoxy molding compound (EMC). The leadframe device 100 comprises a substrate, in the form of a leadframe 102, having a plurality of internal leads 104 for bonding a plurality of wires 106 to a chip 108. The chip 108 is mounted on a die pad 110, which is usually centrally located and surrounded by the internal leads 104. The wire-bonded leadframe is then packaged by encapsulation with EMC 112, which is shown partially molded onto the leadframe device 100. The encapsulation should seal off the plurality of internal leads 104, bonded wires 106, the chip 108 and the die pad 110 to protect the encapsulated material from environmental interference such as electromagnetic waves, contamination, and mechanical, thermal and electrical shock. The leadframe 102 further comprises a plurality of external leads 114 which are not sealed by EMC 112. After each unit of the encapsulated leadframe is separated from a carrier rail 116 by cutting, the plurality of external leads 114 may be formed into different shapes as required for connection to external devices.
As the semiconductor device comprises various components assembled together, the quality and reliability of the leadframe are important to achieve certain criteria, such as the bondability of internal leads 104 with bonding wires 106, the solderability of external leads 114 with external devices, and the adhesion of EMC 112 with the surfaces of the leadframe 102. To enhance quality and reliability, multiple coatings of material are typically plated onto leadframes. One form of these leadframes is commonly referred to as pre-plated leadframes (PPF). In a typical embodiment, PPFs comprise a base material such as copper (Cu) or a copper alloy, which is plated with a number of layers of different metals, such as nickel (Ni), and noble metals such as palladium (Pd) and gold (Au).
Since there is a worldwide trend for the elimination of lead (Pb) from semiconductor packaging, PPF technology is gaining increasing popularity as an environmentally-friendly technology. This is because PPF leadframes are plated with palladium (Pd) instead of silver (Ag) and tin/lead (Sn/Pb). Palladium is useful to protect the underlying plating layers and serves to promote bondability and solderability. The plating may cover the entire leadframe before the packaging process so that an automated packaging process could be carried out easily. A layer of nickel or nickel alloy is plated between the copper base material and the palladium layer to provide the leadframe with corrosion resistance by impeding the diffusion of copper to the surface of the leadframe. Copper reacts with air to form copper oxide, which adversely affects the quality and reliability of the leadframe.
FIG. 2 is a schematic sectional view of a three-layered pre-plated leadframe of the prior art. It shows a typical three-layered PPF or leadframe 120 comprising a substrate 122 made of a base material such as copper or a copper alloy.
A layer of nickel 124 is plated over the substrate 122 for impeding diffusion of copper from the base material. It is also the layer to which solders and wire bonds ultimately adhere.
Two layers comprising noble metals, such as palladium 126, followed by gold 128, are then plated in sequence to act as facilitators of wire bonding and soldering. They provide protection for the underlying nickel layer from oxidation and promote solderability owing to their fast diffusion into solders.
As manufacturers seek to make the plating layers ever-thinner to save costs, it is inevitably easier for the copper base material to out-diffuse to the surface of the leadframe through material defects such as inherent porosity in the form of pin-holes and grain boundaries. In FIG. 2, the migration of copper from the substrate 122 to the surface of the three-layered leadframe 120 is shown. When the diffused copper 130 reaches the surface of the leadframe 120, it will oxidize upon contact with the ambient air. As a result, degradation of wire bondability and solderability occurs on the surface of the leadframe 120.
Despite its favorable properties, the use of nickel increases the cost of manufacturing such leadframes in terms of both raw materials and production cycle time. Another benefit from reducing nickel thickness is that a thinner nickel layer tends to promote increased surface roughness of the base leadframe, which is usually deliberately introduced to enhance the adhesion of EMC to the leadframe. However, if the use of nickel is reduced, reliability issues arise because of degradation of bondability and solderability due to increased diffusion of copper to the surface of the leadframe. Accordingly, the invention seeks to reduce at least the use of nickel in the plating layers by introducing an additional obstacle separating the base material and nickel layer to impede the out-diffusion of copper from the base material through the multiple plating layers.